The invention relates to a process for the production of a polyamide shaped article by means of moulding from the melt. The polyamide""s crystallisation behaviour plays a very important part in moulding from the melt by means of, among other methods, injection-moulding, extrusion and spinning. For economic reasons in particular, the crystallisation rate and the crystallisation initiation are important in this context. For this reason nucleating agents are added to most polyamide compositions. Usually very finely distributed inorganic substances are used for this purpose. The most frequently used inorganic nucleating agents are microtalc and silica. The use of these nucleating agents however imposes very high demands on their good dispersion in the polyamide. Organic compounds also find application in some commercially available polyamide compositions for injection-moulding applications, in particular in PA-6.6 compositions, in which Na-phenylphosphinate is used. Polymers can also be used as nucleating agents, providing their melting points lie above that of the polyamide composition. JP-A-58201844 for example discloses the use of polyamide-4.6 as a nucleating agent for polyamide-6 or -6.6. The polyamide-6 or -6.6 composition containing 0-5 wt. % of a polyamide-4.6 powder with a particle size of less than 100 xcexcm is to this end heated to a temperature above the melting temperature of PA-4.6 and subsequently spun or processed by means of injection-moulding. This process results in an increase in the crystallisation temperature of about 5-10xc2x0 C., depending on the concentration of polyamide-4.6 used. A serious objection to this process is that the moulded parts obtained have a very high content of visible inhomogeneities, which is disadvantageous for the mechanical and optical properties and which for example makes their use for the production of film rather unattractive.
The inventors have attempted to meet all the objections to the processes mentioned above and to develop a process that can be carried out in a simple manner and that can be used for all techniques for moulding from the melt.
They have succeeded in this by first mixing the polyamide-4.6 into a minor amount of the polyamide with a lower melting point, at a temperature above the melting point of polyamide 4.6, and subsequently processing a mixture of the composition thus obtained and the major part of the polyamide with the lower melting point into the desired shaped article.
The process according to the invention for the production of a polyamide shaped article from a polyamide composition, comprising 0.01-5 wt. % polyamide-4.6 and 99.99-95 wt. % of a polyamide with a melting point lower than that of polyamide-4.6, the wt. % being relative to the overall amount of polyamide, by means of moulding from the melt is characterised as further indicated in claim 1.
xe2x80x98Polyamide-4.6xe2x80x99 is understood to be a polyamide in which at least 50% of the repeating units are tetramethylene adipamide units. Preferably at least 75%, even more preferably at least 90%, of the repeating units are tetramethylene adipamide units. Polyamide-4.6 can be obtained through polycondensation of tetramethylene diamine and adipic acid or its adduct, optionally in the presence of other polyamide-forming monomers, for example E-caprolactam, a different diamine, for example hexamethylene diamine, or a different carboxylic acid, for example isophthalic acid or cyclohexane dicarboxylic acid. Polyamide-4.6 and its preparation are described in, for example, the Encyclopaedia of Polymer Science and Engineering, Vol. 11, pp. 315 ff (1988), and in the works referred to therein. Polyamide 4.6 is commercially available under the tradename STANYL(copyright), produced by DSM.
A xe2x80x98polyamide with a lower melting pointxe2x80x99 is understood to comprise any homopolyamide, copolyamide or mixture of homopolyamides, mixture of a homopolyamide and a copolyamide or mixture of copolyamides having a melting point below the melting point of the polyamide 4.6 as defined above. Examples of these polyamides with a melting point lower than that of polyamide 4.6 are to be found for example in the aforementioned Encyclopaedia. Commercially available are for example polyamide-6, polyamide-11, polyamide-12, polyamide-6.6, the polyamide based on metaxylilene diamine and adipic acid, copolyamides based on caprolactam, hexamethylene diamine and aromatic dicarboxylic acid, or based on methylpentamethylene diamine, hexamethylene diamine and one or more dicarboxylic acids.
The polyamide composition (A+B) for the process of the invention contains at least 0.01 wt. %, preferably at least 0.1 wt. %, polyamide-4.6. A concentration of less than 0.01 wt. % has no appreciable effect. The polyamide-4.6 content is preferably less than 5 wt. % because at higher contents the risk of gel formation in the composition increases, and the effect of a further increase in the polyamide-4.6 concentration on the crystallisation behaviour is negligible.
The molecular weight of the polyamide with the low melting point in the composition (A+B) may vary within a wide range and is predominantly dependent on the type of moulded article and the moulding technique. For example, a higher molecular weight, for example an Mn of approx. 20,000, will preferably be used for extrusion, while for injection-moulding of thin-walled objects a lower molecular weight, Mn of approx. 13,000, will preferably be used.
The molecular weight of the polyamide-4.6 is also of minor importance. The crystallisation behaviour of a composition of polyamide-6 and polyamide-4.6 of a low molecular weight does virtually not differ from that of a composition with the same concentration of polyamide-4.6 of a high molecular weight. For practical reasons it is preferable for the polyamide with a low melting point in B to have a lower molecular weight than this polyamide in A.
In another form, which however in some circumstances is less preferably, the lower melting polyamide of composition B may be different from the lower melting polyamide A. The advantage is that with one kind of masterbatch the crystallisation behaviour of different polyamides can be influenced. However in such a case in general the fraction of B in (A+B) should be chosen as low as possible to avoid negative effects on the mechanical properties.
In the process according to the invention the composition is shaped from the melt using the usual techniques, such as injection-moulding, extrusion, melt spinning and rolling, under the usual conditions for the polyamide concerned. Moulded articles obtained using the process are for example film, fibre, extruded articles such as sheets, rods and tubes and injection-moulded articles.
A particular characteristic of the composition (A+B) according to the invention besides a higher temperature, at which the crystallisation starts already during cooling, is that the crystallisation takes place across a wide range of temperatures if the cooling takes place at a constant rate, as is the case for example in differential scanning calorimetry (DSC). This particular behaviour could be explained by the facts that, on the one hand, the presence of polyamide-4.6 accelerates the initiation of the crystallisation and, and on the other hand, the crystallisation rate as such is not increased. If polyamide-6 is the polyamide with the lower melting point, two peaks, at approx. 203 and approx. 194xc2x0 C., can even be distinguished in the DSC scan, which could point to the occurrence of two crystallisation forms.